Nowadays, many wireless communication schemes are being proposed as candidates for high-rate mobile communication. Among them, an OFDM (Orthogonal Frequency Division Multiplexing) scheme is being esteemed as the most prominent next-generation wireless communication scheme. The OFDM scheme is expected to be used in most future wireless communication schemes. The OFDM scheme is also adopted as the standard in an IEEE 802.16 WMAN (Wireless Metropolitan Area Network) called a 3.5th generation (3.5G) technology.
The OFDM scheme uses multi carriers to transmit data. That is, the OFDM scheme is a kind of multi-carrier modulation (MCM) scheme that parallelizes serial input symbols and modulates the parallel symbols with a plurality of orthogonal subcarriers (i.e., subchannels) prior to transmission.
In a cellular wireless communication system, the quality of a communication between a mobile station (MS) and a base station (BS) degrades due to poor channel conditions caused by the geographical conditions in a cell, the distance between an MS and a BS, or the movement of an MS. For example, even in a coverage area of the BS, propagation shadow areas may be caused by closed buildings such as offices and houses. If the MS is located in the propagation shadow area, the BS may not perform a smooth communication due to the poor channel condition with the MS.
Thus, the wireless communication system may provide a femto-cell service for providing a high-rate data service while overcoming the problem of a service in the propagation shadow area. The femto cell is a cell created by a compact BS that is located in a building such as an office or a house and is connected to a mobile communication core network through a broadband network. The compact BS is a low-power BS installed by a user. The compact BS may be called a micro BS, a self-configurable BS, an indoor BS, a home BS, or a femto BS. Hereinafter, the compact BS will be referred to as a femto BS.
A handover interruption time is one of the most important performance indicators. In a handover process, an MS releases a connection with a serving BS and establishes a connection with a target BS. Herein, a time period from the time point of releasing the connection with the serving BS to the time point of receiving data packet from the target BS through the connection with the target BS may be defined as the handover interruption time. The handover interruption time may be regarded as an important performance indicator because it determines the possibility of service continuity, that is, seamless services. The handover interruption time is affected by the time taken to perform a handover process between the MS and the target BS.
In a handover of an MS to a target BS, the 802.16e Mobile WiMAX system exchanges a ranging request message and a ranging response message between the MS and the target BS to update a connection identifier (CID) to be used in the target BS by the MS and perform mutual authentication between the BS and the MS. Accordingly, downlink (DL) and uplink (UL) service traffics are not transmitted to the MS during the handover of the MS to the target BS. This may be regarded as a service interruption from the user's standpoint.